This invention relates to a method of applying a variety of coating solutions to supports made of plastic film, paper, or the like, and to an apparatus for practicing the method. More particularly, the invention relates to a method in which, in the manufacture of photographing photo-sensitive materials (such as photographic films and papers) and magnetic recording materials (such as magnetic recording tapes), coating solutions (such as photographic emulsions and magnetic solutions) are applied to belt-shaped supports (hereinafter referred to merely as "webs", when applicable), and an apparatus for practicing the method.
A method in which a coating solution is applied to a support made of plastic film or paper and dried to form a certain product has been extensively employed in the field of manufacture of photographic films and papers, magnetic materials, pressure-sensitive recording sheets and heat-sensitive recording sheets, and thin film macromolecular materials.
One example of a method which is extensively employed for coating a web with a liquid-phase coating compound is a method using a multi-layer slide bead coating apparatus which has been described in the specification of U.S. Pat. No. 2,761,791 to Russell et al. In the method, a plurality of coating solutions flow down a sloped surface (or slide surface), to form a bead at the lower end of the slide surface where the coating solutions meet the web which is moving thereby, so that coating is achieved with the aid of the bead thus formed.
A single-layer or multi-layer coating apparatus used in a slide bead coating method, extrusion bead coating method, hopper coating method and curtain coating method will be described with reference to FIG. 1. In the apparatus, a coating solution 2 is supplied into a coating head 1 by a metering pump 9. More specifically, the solution 2 is first supplied into a cavity 5 through a coating solution supplying pipe 3 and a coating solution supplying inlet 4. The solution 2 is then delivered through a slot 6 to a slot opening 7, where it is applied to a web 8. The apparatus generally employs a differential pressure chamber 11 for achieving a stable coating operation. In the apparatus, the uniformity in thickness of the coating in the widthwise direction of the web depends on the precision of the slot 6 and the slot opening 7. Accordingly, it is essential to form the slot opening 7 with extremely high accuracy.
However, the conventional coating head suffers from serious drawbacks. In the cavity 5, the slot 6 and/or the slot opening 7, the coating solution 2 contained therein forms residue deposits like cake, thus making it difficult to form a coating uniform in thickness in the widthwise direction of the web. Furthermore, some of the residues thus formed may stick to the coating or may form streaks on the coating.
It is quite difficult to remove the residue through the slot opening. Accordingly, it is necessary to disassemble and wash the coating head for removal of the residue. This cleaning process takes a relatively long time.
If the coating operation is carried out for a long period of time, in the head, especially in the cavity 5, the flow of the solution 2 is liable to be retarded. Where the solution is photographic emulsion, a thickener whose viscosity increases with time such as gelatin bridged material formed by a hardening agent is formed. Where the coating is magnetic material, the dispersant will condense, thus forming the residue which can accumulate in the cavity 5 or can clot the slot opening. The residue thus formed makes the thickness of the coating non-uniform in the widthwise direction of the web and reduces the quality of the product.
As was described above, the solution 2 is supplied into the cavity in the head 1 through the tube 3 by the pump 9, and is then delivered through the slot 6 to the slot opening 7, through which the coating solution is distributed on the web. That is, in this operation, the solution is abruptly spread over the entire width of the coating head. Therefore, the coating solution flow is liable to be retarded at the edges of the flow path or where the flow path is abruptly expanded or contracted. It is very difficult to design a cavity which, when a coating solution is applied to the web, will minimize residue formation. Especially when a general purpose coating head is required, it is impossible to design the head so as to prevent the retardation of the flow therein.
To eliminate the above-described difficulties, a coating apparatus having a coating head in which the cavity spreads radially in the form of a triangle from the coating solution supplying inlet towards the slot opening has been proposed (cf. Japanese Patent Application Publication No. 48146/1984). In addition, for the same purpose, a method has been proposed in which, to prevent the retardation of the coating solution flow at the ends of the cavity, auxiliary solution supplying tubes are provided at the cavity ends to inject the solution thereinto and discharge it therefrom (cf. U.S. Pat. No. 4,623,501).
However, in the conventional coating method and apparatus, it is still difficult to substantially eliminate the above-described difficulties that the coating is not uniform in thickness over the entire width of the web and foreign matters such as the residue of the solution sometimes becomes part of the coating.